The goal of this proposal is to gain insight into the molecular processes controlling alternate ab/gd lineage commitment. This is of critical importance not only because the divergence of T cells into functionally distinct ab and gd lineages is essential for normal immune responses, but also because differentiation of multipotential precursor cells into distinct cell types is a fundamental yet poorly understood process common to all multicellular organisms. Despite much effort, relatively little insight has been gained into the developmental cues that determine whether an immature thymocyte will adopt the ab or gd lineage. Importantly, we recently provided compelling evidence for a signal strength model of lineage commitment which posits that weak signals promote commitment to the ab lineage while comparatively strong signals promote commitment to the gd lineage, irrespective of the TCR complex from which they originate. In pursuing these studies, we exploited an ideally suited gd-TCR transgenic model (KN6), which has a known ligand whose expression can be manipulated to alter the nature of the resultant TCR signal. We have demonstrated that the KN6 gd TCR complex requires engagement by ligands to promote adoption of the gd fate. This finding was highly controversial when first reported, but is now gaining support as other TCR/ligand pairs are examined. We further proposed that the differences in signal strength that alter fate are dependent upon differential activation of the ERK-early growth response (Egr)-Id3 pathway. In the current proposal, key features of the signal strength model will be tested as follows. In Aim 1, we will assess the affinity of the selecting ligands, as well as whether they regulate fate instructively. Based on our observation that differential activation of ERK influences fate determination, Aim 2 is focused on investigating the importance of the amplitude and duration of ERK activation on commitment. In particular, we will investigate the function of ERK DEF domains, a key domain in mediating the effects of a long ERK signal. Finally, we have shown that Id3 is a downstream target of strong signals that promotes the gd fate and antagonizes the ab fate in our KN6 model, in addition to perturbing the development of gd lineage cells in non-Tg Id3-deficient mice. Hence, in Aim 3, we will elucidate the molecular mechanism whereby Id3 regulates gd T cell development. Our investigation of molecular effectors controlling T lineage commitment is of fundamental importance not only for thymocyte development, but also for other developmental processes, since control of cell growth and differentiation is a recurring theme in development and transformation.